Paper feed and exposure synchronizer

ABSTRACT

A synchronizer for an electrophotographic device is provided wherein an electronic flash exposure is initiated in response to the position of a movable photosensitive surface to be exposed and the position of a paper feed device for feeding receiver sheets onto which a toned image is to be placed. The paper feed device operates cyclically during movement of the photosensitive surface but is enabled to feed paper only in response to the proper position of an image on the photosensitive surface and is disabled in response to position of the paper feed device itself. This invention is particularly applicable in an electrophotographic device in which an endless photoconductive web is used to form a toner image which image is transferred in proper time relation to a receiver sheet fed by the paper feeding device.

Reesen [111 3,744,900 1 July 10,1973

PAPER FEED AND EXPOSURE SYNCHRONIZER Jorgen Reesen, Pittsford, N.Y.

Eastman Kodak Company, Rochester, N.Y.

Filed: Sept. 27, 1971 Appl. No.2 184,227

Related U.S. Application Data Continuation of Ser. No. 23,705, March 30,1970, abandoned.

Inventor:

Assignee:

U.S. Cl 355/16, 355/3, 355/99, 355/108 Int. Cl. G03b 27/00 Field ofSearch 355/3, 16, 99, 100, 355/102, 104, 106-110 References Cited UNITEDSTATES PATENTS Primary ExaminerSamuel S. Matthews Assistant Examiner-E.M. Bero Attorney-R. W. Hampton 57 ABSTRACT A synchronizer for anelectrophotographic device is provided wherein an electronic flashexposure is initiated in response to the position of a movablephotosensitive surface to be exposed and the position of a paper feeddevice for feeding receiver sheets onto which a toned image is to beplaced. The paper feed device 0perates cyclically during movement of thephotosensitive surface but is enabled to feed paper only in response tothe proper position of an image on the photosensitive surface and isdisabled in response to position of the paper feed device itself. Thisinvention is particularly applicable in an electrophotographic device inwhich an endless photoconductive web is used to form a toner image whichimage is transferred in proper time relation to a receiver sheet fed bythe paper feeding device.

8 Claims, 3 Drawing Figures 12/1968 Klosky et al. 355/110 X PAramiu m3.744.900 SHEET 1 if 2 DEVELOHNG STATION ATTORNEYS PATENTED JUL 1 FIG.2

STEH 2 0F 2 EX'POSURE SIGNAL SENSOR OUTPUT /76 J ARMING cmcun OUTPUTVACUUM FINGER CYCLE k A A SENSOR OUTPUT AND GATE OUTPUT A VACUUM ONSENSOR OUTPUT L VACUUM OFF SENSOR OUTPUT JORGEN REESEN INVENTOR.

BY we J/JMI/M ATTORNEYS 1 PAPER FEED AND EXPOSURE SYNCHRONIZER Thisapplication is a continuation of application Ser. No. 23,705 filed3/30/70 now abandoned.

CROSS-REFERENCE TO RELATED APPLICATIONS Reference is made to copending,commonly assigned U.S. Application Ser. No. 19,999, entitled MACHINEPROGRAMMER to Lional R. Hickey and Ellsworth J. McCune and U.S.Application Ser. NO. 19,644 entitled MAGNETICALLY CONTROLLED MACHINEPROGRAMMER" to Lional R. Hickey, both filed Mar. 16, 1970.

BACKGROUND OF THE INVENTION l. Field of the Invention This inventionrelates to a synchronizer and more particularly to a device forsynchronizing flash exposure and paper feed operations so that an imagemay be formed on a photosensitive member and transferred in proper timerelation to a receiver sheet for accurate registry.

2. Description of the Prior Art In one form of electrophotographicprocess, an endless photoconductive member, such as a drum or an endlessweb moves continuously through a series of electrophotographic stationsto form a toner image from an original, which toner image is transferredto a receiver sheet to make a copy of an original. Typically, thephotoconductive member first is advanced past a charging station whichplaces a generally uniform electrostatic charge on the photoconductivemember, such as by corona discharge. Next it passes an exposure stationwhere it is exposed to a pattern of radiation corresponding to anoriginal to be copied. This discharges a photoconductive member in theexposed areas to form an electrostatic latent image. This image is tonedat a developing station to form a toner image. As a photoconductivemember continues along its endless path, the toner image can be utilizedin any one of several ways but is commonly transferred to a receiversheet at a transfer station to form a copy of the original. At asubsequent station any residual toner particles remaining on thephotoconductive member are cleaned therefrom so that the photoconductivemember may be recycled through the stations just described.

With such electrophotographic apparatus, it is clear that the operationof each station along the path of the photoconductive member must beperfectly synchronized with the movement of the photoconductive memberso that each function is performed at the proper time and in the propersequence to make a satisfactory print. If any one function is advancedor retarded slightly with respect to movement of the photoconductivemember, the resulting image may be imperfectly registered on thereceiver sheet. Such synchronization can be accomplished by use of theshift register disclosed in commonly assigned, copending U.S.Application Ser. No. 19,999, entitled MACHINE PRO- GRAMMER and U.S.Application Ser. No. 19,644, entitled MAGNETICALLY CONTROLLED MA- CHINEPROGRAMMER, both filed on Mar. 16, 1970. In the prior art, variousattempts have been made to synchronize sheet feeding with the movementof the photoconductive surface. For example, U.S. Pat. Nos. 3,060,108;3,062,109; 3,187,651 and 3,256,009 each disclose an electrophotographicapparatus in which a copy sheet is fed in synchronization with rotationof a photoconductive drum. In U.S. Pat. No. 3,148,601 the document feed,as well as the copy sheet feed, is operspeeds to control spacing betweenimages.

In each instance the exposure is made and the copy sheet is fed inaccordance with respective preselected positions of the photoconductivemember. This method does not provide adequate registry of image and copysheet unless the inaccuracies inherent in or developing during use ofthe aforementioned shift register or equivalent controldevices arestrictly minimized.

In prior art devices it is necessary to provide both complex exposurecontrol devices to expose the image in accurate location on thephotoconductive member, and complex paper feed the control meansprovides a third signal as an output signal to activate the flash. Inaddition, means are provided to enable the paper feed mechanism so thatit feeds a sheet of receiver paper during a subsequent cycle so that thereceiver sheet reaches the transfer station at the same time as a tonerimage. The paper feed mechanism is subsequently disabled in response tothe position of the paper feed mechanism in its cycle.

More particularly, in a preferred embodiment signals in response to theposition of a photoconductive web, movable along an endless path, areprovided by a shift register such as those disclosed in either of theabovementioned, commonly assigned, copending U.S. Applications. When acharged portion of the photoconductive web approaches a proper positionfor exposure, the shift register provides a signal to an arming circuitwhich in turn provides a first signal to an AND gate. When a paper feedmechanism, such as a vacuum or suctionfinger is in the exact positionwith respect to the charged portion of the photoconductive web itprovides a second signal to the AND gate. The AND gate now sends anoutput signal to a flash trigger circuit which in turn provides anoutput signal to trigger the electronic flash. The output signal of theflash trigger circuit also resets the exposure arming circuit. The shiftregister also causes the constantly operating paper feed means to beenabled, as by providing a vacuum to the vacuum finger for separatingindividual receiver sheets from a stack for feeding seriatim to thetransfer station. This is accomplished by an output signal from theshift register to a vacuum control circuit which operates a solenoidvalve connected to a vacuum source. The vacuum is turned off when thesuction fingers reach the appropriate position for feeding the paper tothe transfer station by a signal to the vacuum control circuit inresponse to the position of the vacuum finger which causes the valve toclose so that the vacuum at the finger is interrupted at precisely theright time. This reduces the possibility of paper jams and permits theuse of lighter weight paper.

Thus, the invention provides an improvement in accuracy of registrybetween exposure and copy sheet. Also, if the relation of phase betweenweb drive and paper feed should change somewhat by wear of belts, gearsor the like, or through service disassembly and reassembly, replacementof parts or the like, exposure and paper feed will be delayed oradvanced by equivalent amounts, thereby maintaining accurate registry ofimage and copy sheet.

Additional advantages of this invention will become more apparent fromthe description which follows and taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic, fragmentary,perspective view of an electrophotographic device showing details of thesynchronizer of this invention;

FIG. 2 is a series of energy-time curves showing the time relationshipbetween various signal pulses for synchronization of the electronicflash and paper feed mechanism of FIG. I; and

FIG. 3 is a series of energy-time curves showing the time relationshipbetween the signal sensors for enabling the paper feed mechanism and fordisabling it.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with thisinvention a synchronizer is provided so that differentelectrophotographic operations are performed on a photoconductive memberin proper sequence and in timed relation. A photoconductive member, suchas a photoconductive belt 2, moves along an endless path past a chargingstation 3, an exposure station 4, a developing station 6, around a driveroller 8 and past support rollers 10 and 12 at a transfer station 14.The belt then can continue past a cleaning station (not shown), ifdesired, then be recycled past the stations previously described.

Exposure of a charged portion of photoconductive web 2 is made, as by apair of electronic flash units 16 which are positioned to reflect animage from an original 18 through a lens 20 onto the charged portion.The charge is dissipated in the exposed areas to form an electrostaticimage. This image then passes through developing station 6 to form atoner image and then proceeds to transfer station 14. A supply ofreceiver sheets 22 are sequentially fed from a stack, as by a reciprocalvacuum finger 24, to a pair of transport belts 26 and 28 which bring thesheet 22 into contact with the toner image on web 2 to transfer theimage to the receiver sheet. After transfer, the receiver is separatedfrom the web (by means not shown) and the toner image thereon may befused, if necessary. Vacuum finger 24 is connected to a flexible hose 30to a source of vacuum (not shown). Hose 30 includes a valve 32 forintermittently connecting vacuum finger 24 to the source of vacuum in amanner to be described. For convenience of illustration, only one vacuumfinger has been illustrated, but it should be understood that aplurality of vacuum fingers can be provided to handle any width receiversheets.

Photoconductive web drive roller 8 is driven by a power source, such asa motor 34, connected to shaft 36 which in turn is connected by means ofbelt 38 and i the photoconductive member required for making a singleprint. As more fully disclosed in the above mentioned, commonly assignedpatent applications, the shift register is divided into a plurality ofsectors 44 each of which includes a lamp, such as lamp 46, which isenergized to light the sector, by means shown in the copendingapplications, to sequentially initiate the electrophotographicoperations on each print sector as the print sectors move through thevarious stations. A plurality of sensors are provided around shiftregister 42 to initiate each of the operations, most of which have beenomitted in FIG. I for clarity of illustration. Drive roller 8 ismechanically connected to vacuum finger 24 which moves through acyclical path in response to movement of the photoconductive web.Conveniently, the ratio of the drive connection which couples the web tothe vacuum finger is such that the vacuum finger goes through one cyclefor each advancement of a print sector through the exposure stationwhether paper is fed or not, as described below. A mechanical connectionis illustrated which includes a relatively large timing gear 47 on shaft36 which through timing belt 48 drives a smaller timing gear 49 mountedon shaft 50. A large timing gear 51 isgdriven by shaft 50 and throughtiming belt 52 drives tiining gear 53 to rotate shaft 54. The rotationof the shaft 54 is converted into a reciprocal motion within drive box55 which may include a bell crank (not shown) which is connected by anarm 56 to suction finger 24, as shown.

Electronic flash units 16 must be operated in proper time sequence withthe'reciprocal movement of vacuum finger 24 so that the electrostaticimage formed on the print sector is developed and received at transferstation 14 at the same time that a receiver sheet 22 reaches thetransfer station so that a toner image is properly positioned on thereceiver sheet. To this end, a lighted sector 44, upon rotation of theshift register and advancement of the web to a position somewhat inadvance of the point of exposure, is aligned so that a sensor 66provides a signal to exposure arming circuit 68 which in turn provides acontinuous signal to AND gate 64. By way of example, the exposure armingcircuit 68 may include a flip-flop which switches to a first state toapply a high level signal to the AND gate 64 in response to an inputsignal from the sensor 66 and switches to a second state to apply a lowlevel signal to the AND gate 64 in response to a reset signal from theflash trigger circuit 70. Additionally, photoelectrical signal means,such as sensor 62, located peripherally to disc 60 on shaft 54 isactivated by a signal from a light 58 shining through a slot 59 in thesidewall of disc 60 at the appropriate position of finger 24, to providea signal pulse to AND gate 64.

If the shift register were to have a sensor to directly trip theexposure, variations in accuracy of manufacture of the sector rotor, aswell as variations between the individual sector light bulbs flux due toindividual bulb and circuit characteristics, would cause advancement orretardation of sensor pulse as each individual sector activated thesensor. The result would be a cyclically repetitive set of registryerrors, aggravated by the slower angular rotation of the shift registercompared to the synchronizer. The same is true of the vacuum cut-off.

Upon receiving bogh signals, AND gate 64 provides an output signal toflash trigger circuit 70. The output from flash trigger circuitactivates a high voltage member and the position of the vacuum finger toassure 5 that exposure occurs when both the photoconductive web and thevacuum finger are in the proper positions.

A better understanding of the signal sequences may be had by looking atFIG. 2. Each time a print sector of photoconductive web 2 is in properposition for exposure, shift register 42 is positioned so that a lightedsector 44 causes sensor 66 to provide output signal 74 which is a shortpulse as shown in FIG. 2. This signal causes arming circuit 68 toprovide a longer and continuous output high level signal 76 to AND gate64. For

each cycle of reciprocal vacuum finger 24 sensor 62 provides an outputsignal 78, and this is true whether or not a vacuum is being applied tothe vacuum finger. One of these pulses 78 occurs during an output signal76 of arming circuit 68 which causes AND gate 64 to give an outputsignal 80. Thus the output signal causing the exposure occurs when thepaper feeding mechanism is at a point in its cycle corresponding tolater delivery of a copy sheet to a position on the photoconductive webcongruent with the location of the exposure made.

A vacuum-on sensor 82 provides an output signal when the correspondinglighted sector 44 of the shift register has rotated to be aligned withthat sensor. As

shown in FIG. 3, an output signal in the form of a pulse 84 is providedto a vacuum control circuit 86 which in turn provides an output signal88. Circuit 86 has its own internal holding device and remains on afterthe signal pulse 84 disappears. The circuit 86 may include a flip-flopwhich switches to a first state to energize a solenoid 90 in response toan input signal from the sensor 82 and switches to a second state tode-energize the solenoid 90 in response to a signal from the sensor 91.The vacuum-on sensor is aligned with respect to the rotating shiftregister 42 to turn on the vacuum to the feed finger 24. The timing ofsignal 84 is not critical and will be sufficiently accurate as long asvacuum is established during the downward travel stroke of the paperfeed vacuum finger. The vacuum control circuit by means of output signal88 energizes a solenoid 90 to open valve 32 so that a vacuum appears atvacuum finger 24. As stated above, the feed finger goes through itscycle repetitively, once per print sector advancement of the belt, andif valve 32 is opened it picks up a sheet of receiver paper 22 when itis at the bottom of its stroke. During upward motion of the feed fingera sheet of receiver paper is delivered to paper transport belts 26 and28. A sensor 91 is positioned adjacent disc so that as soon as theleading edge of the receiver 5 sheet 22 which is being fed is firmlyheld by belts 26 and 28, slot 59 is positioned so that a light 61 shineson sensor 91. An output signal 92 from sensor 91 deactivates vacuumcontrol circuit 86 to close valve 32 to shut off the vacuum. Preciseshut-off of the vacuum avoids exertion of reverse forces on the receiverpaper at the most critical point in the paper feeding cycle. Thus,registry can be improved; creasing or buckling of the paper can beavoided, reducing the risk of paper jams and allowing the use of lighterweight paper.

From the foregoing, the advantages of this invention are readilyapparent. A synchronizer has been provided in an electrophotographicapparatus for causing exposure of a charged portion of a photoconductiveweb in proper sequence with the cyclical movement of a paper feedingdevice such as a vacuum feed finger. Furthermore, the vacuum to the feedfinger is turned on in accordance with the position of the advancingelectrostatic image on the web and is turned off in accordance with theposition of the feed finger itself so that it occurs at a time when theleading edge of the receiver sheet being fed is gripped by a separatetransport means. This reduces the possibility of paper jams and permitsthe use of lighter weight paper. Also, accurate registration is providedeven though there is wear of gears, belts, replacement of parts, etc.

The invention has been described in detail with particular reference toa preferred embodiment thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

I claim:

1. In electrophotographic apparatus having an electrophotosensitive web,means for charging a plurality of spaced image receiving portions of theweb and adapted to have a plurality of electrostatic images of anoriginal formed thereon respectively, the web being movable along afirst predetermimned path, and means adapted to transfer informationcontained in each such electrophotostatic image to receiver sheets inimage transferring relation therewith respectively, the combinationcomprising:

a. a plurality of work stations disposed adjacent said first pathincluding means for charging a portion of the web, an actuable exposurestation operative when actuated for exposing such charged portion of thephotosensitive web to a light image to form an electrostatic image onthe web, a developing station for contacting such electrostatic imagewith a medium which adheres to the web in a configuration defined bysuch electrostatic image to form a toner image and an actuable sheetfeeder movable through a predetermined sheet feeding cycle and operativewhen actuated for feeding a receiver sheet into an image transferingrelation with respect to toner image on the web for permitting thetransfer of at least a portion of the medium in an image configurationto such receiver sheet;

b. means defining a second predetermined path;

c. means coupled to said web moving means and movable along said secondpath for sequentially (1) providing a first signal when a chargedportion of the web moves to a position wherein it is capable of beingexposed in said exposure station, and (2) actuating said sheet feeder,in timed relation to movement of the web past predetermined positionsalong said first path; and

d. exposure station actuating means comprising:

i. means coupled to said sheet feeder for providing a second signal inresponse to said sheet feeder being in a predetermined position in saidsheet feeding cycle; and

ii. means responsive to said first and second signals for actuating saidexposure station causing said exposure station to expose a chargedportion of the web to an image, whereby image exposure on the web issynchronized with the operation of said sheet feeder.

2. The invention as claimed in claim 1 wherein sequential meansincludes:

means for producing a third signal indicative that a charged portion ofthe web has entered said exposure station; and

exposure arming means effective in a first condition in response to saidthird signal to provide said first signal and in a second condition toprevent said exposure arming means for producing said first signal.

3. The improvement claimed in claim 1 wherein:

said third signal providing means is an AND gate.

4. The invention as claimed in claim 1 further including:

a flash trigger circuit responsive to said third signal from said ANDgate to expose an image of an original on the web and to produce afurther signal to said arming means causing it to be in said second Icondition.

5. ln electrophotographic apparatus having an electrophotosensitivemember having a plurality of discrete electrostatic images ofan'original formed thereon in predetermined spaced relationship withrespect to each other, and means adapted to transfer informationcontained in such electrostatic images to receiver sheets, thecombination comprising:

a. a plurality of work stations including a charging station forcharging a portion of the member, an actuable exposure station operativewhen actuated for exposing a charged portion of the member to a lightimage of an original for forming an electrostatic image of such originalon the member, a developing station for contacting such electrostaticimage with a medium which adheres to the member in a configurationdefined by such electrostatic image, and an actuable sheet feedermovable through a predetermined sheet feeding cycle and operative whenactuated for feeding a receiver sheet into an image transferringrelationship with respect to an electrostatic image on the member forpermitting the transfer of at least a portion of the medium in an imageconfiguration to such receiver sheet;

b. means coupled to said sheet feeder and the member for moving theelectrophotosensitive member along a first endless path relative to saidplurality of actuable work stations;

c. means defining a second predetermined path;

d. means coupled to the member moving means and movable along saidsecond path for sequentially (1) providing a first signal when a chargedportion of the member moves to a position wherein it is capable of beingexposed to a light image at said exposure station, and (2) actuatingsaid sheet feeder, in timed relation to movement of the member pastpredetermined positions along said first path; and

e. exposure station actuating means comprising:

i. means defining a third predetermined path;

ii. means coupled to said sheet feeder and movable along said third pathfor providing a second signal in response to said sheet feeder movingthrough a predetermined position in said sheet feeding cycle; and

iii. means responsive to said first and second signals for actuatingsaid exposure station causing said exposure station to expose a lightimage on the member, whereby image exposure on the member issynchronized with the operation of said sheet feeder.

6. The invention as claimed in claim 5 wherein sequential meansincludes:

means for producing a third signal indicative that a charged portion ofthe member has entered said exposure station; and

exposure arming means effective in a first condition in response to saidthird signal to provide said first signal and in a second condition toprevent said exposure arming means from producing said first signal.

7. The improvement claimed in claim 6 wherein:

said third signal providing means is an AND gate.

8. The improvement as claimed in claim 6 further including:

condition.

t l t

1. In electrophotographic apparatus having an electrophotosensitive web,means for charging a plurality of spaced image receiving portions of theweb and adapted to have a plurality of electrostatic images of anoriginal formed thereon respectively, the web being movable along afirst predetermined path, and means adapted to transfer informationcontained in each such electrophotostatic image to receiver sheets inimage transferring relation therewith respectively, the combinationcomprising: a. a plurality of work stations disposed adjacent said firstpath including means for charging a portion of the web, an actuableexposure station operative when actuated for exposing such chargedportion of the photosensitive web to a light image to form anelectrostatic image on the web, a developing station for contacting suchelectrostatic image with a medium which adheres to the web in aconfiguration defined by such electrostatic image to form a toner imageand an actuable sheet feeder movable through a predetermined sheetfeeding cycle and operative when actuated for feeding a receiver sheetinto an image transfering relation with respect to toner image on theweb for permitting the transfer of at least a portion of the medium inan image configuration to such receiver sheet; b. means defining asecond predetermined path; c. means coupled to said web moving means andmovable along said second path for sequentially (1) providing a firstsignal when a charged portion of the web moves to a position wherein itis capable of being exposed in said exposure station, and (2) actuatingsaid sheet feeder, in timed relation to movement of the web pastpredetermined positions along said first path; and d. exposure stationactuating means comprising: i. means coupled to said sheet feeder forproviding a second signal in response to said sheet feeder being in apredetermined position in said sheet feeding cycle; and ii. meansresponsive to said first and second signals for actuating said exposurestation causing said exposure station to expose a charged portion of theweb to an image, whereby image exposure on the web is synchronized withthe operation of said sheet feeder.
 2. The invention as claimed in claim1 wherein sequential means includes: means for producing a third signalindicative that a charged portion of the web has entered said exposurestation; and exposure arming means effective in a first condition inresponse to said third signal to provide said first signal and in asecond condition to prevent said exposure arming means for producingsaid first signal.
 3. The improvement claimed in claim 1 wherein: saidthird signal providing means is an AND gate.
 4. The invention as claimedin claim 1 further including: a flash trigger circuit responsive to saidthird signal from said AND gate to expose an image of an original on theweb and to produce a further signal to said arming means causing it tobe in said second condition.
 5. In electrophotographic apparatus havingan electrophotosensitive member having a plurality of discreteelectrostatic images of an original formed thereon in predeterminedspaced relationship with respect to each other, and means adapted totransfer information contained in such electrostatic images to receiversheets, the combination comprising: a. a plurality of work stationsincluding a charging station for charging a portion of the member, anactuable exposure station operative when actuated for exposing a chargedportion of the member to a light image of an original for forming anelectrostatic image of such original on the member, a developing stationfor contacting such electrostatic image with a medium which adheres tothe member in a configuration defined by such electrostatic image, andan actuable sheet feeder movable through a predetermined sheet feedingcycle and operative when actuated for feeding a receiver sheet into animage transferring relationship with respect to an electrostatic imageon the member for permitting the transfer of at least a portion of themedium in an image configuration to such receiver sheet; b. meanscoupled to said sheet feeder and the member for moving theelectrophotosensitive member along a first endless path relative to saidpluraLity of actuable work stations; c. means defining a secondpredetermined path; d. means coupled to the member moving means andmovable along said second path for sequentially (1) providing a firstsignal when a charged portion of the member moves to a position whereinit is capable of being exposed to a light image at said exposurestation, and (2) actuating said sheet feeder, in timed relation tomovement of the member past predetermined positions along said firstpath; and e. exposure station actuating means comprising: i. meansdefining a third predetermined path; ii. means coupled to said sheetfeeder and movable along said third path for providing a second signalin response to said sheet feeder moving through a predetermined positionin said sheet feeding cycle; and iii. means responsive to said first andsecond signals for actuating said exposure station causing said exposurestation to expose a light image on the member, whereby image exposure onthe member is synchronized with the operation of said sheet feeder. 6.The invention as claimed in claim 5 wherein sequential means includes:means for producing a third signal indicative that a charged portion ofthe member has entered said exposure station; and exposure arming meanseffective in a first condition in response to said third signal toprovide said first signal and in a second condition to prevent saidexposure arming means from producing said first signal.
 7. Theimprovement claimed in claim 6 wherein: said third signal providingmeans is an AND gate.
 8. The improvement as claimed in claim 6 furtherincluding: a flash trigger circuit responsive to said third signal fromsaid AND gate to expose an image of an original on the web and toproduce a further signal to said arming means causing it to be in saidsecond condition.